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Wednesday, January 22, 2014

Objective Type Questions on Power Electronics: Part-11

Power Electronics Objective Type Questions with Answers: Part-11

[1] In a SCR,(a) Gate current is directly proportional to forward breakover voltage(b) As gate current is raised, forward breakover voltage reduces(c) Gate current has to be kept on continuously for conduction(d) Forward breakover voltage is low in the forward blocking state.Answer: B[2] In a SCR, at switch on, there is a sudden increase of current, then(a) The tolerance limit of di/dt varies from 3 to 30A/us(b) di/dt upper limit is 3A/us(c) di/dt rating can be increased by having an inductive load(d) di/dt rating can be increased by a resistive loadWhich of the above statements are true?(1) a, c(2) c, d(3) a, b, c, d(4) a, bAnswer: 1[3] The following PNPN device has a terminal for synchronising purpose(a) SUS(b) DIAC(c) TRIAC(d) Schmitt triggerAnswer: A[4] In single phase full wave controlled rectifier, maximum output voltage is obtained at conduction angle....and minimum at conduction angle...(a) 0°, 180°(b) 180°, 0°(c) 0°, 0°(d) 180°, 180°Answer: A[5] A thyristor is triggered by a pulse train of 5kHz. The duty ratio is 0.4. If the allowable average power is 100W the maximum allowable gate drive power is(a) 100√2 W(b) 50 W(c) 150 W(d) 250 WAnswer: D[6] The consequence of introducing inductance in the load circuit of SCR is(a) Anode current through SCR rises slowly(b) Anode current reached is more than without it(c) SCR has to be derated for anode current(d) SCR's current rating can be raisedWhich of the above statements are true?(1) a, c(2) a, b, c, d(3) b, d(4) a, dAnswer: 4[7] When a SCR is in the forward blocking state,(a) All the 3 junctions are reverse biased(b) The anode and cathode junctions are forward biased but the gate junction is reverse biased(c) The anode junction is forward biased but the cathode and gate junctions are reverse biased(d) The anode and gate junctions are forward biased but the cathode junction is reverse biasedAnswer: B[8] When we need to drive a dc shunt motor at different speeds in both directions ( forward and reverse) and also to brake it in both the directions, which one of the following would you use?(a) A half-controlled SCR bridge(b) A full controlled thyristor bridge(c) A dual converter(d) A diode bridgeAnswer: C[9] The frequency of the ripple present in the output voltage of the 3 phase half controlled bridge rectifier depends on the(a) Firing angle(b) Load inductance(c) Load resistance(d) Supple frequencyAnswer: D[10] A fully controlled thyristor bridge drives a DC motor. The system is capable of(a) Motoring and braking in both directions(b) Only motoring in both directions, no braking(c) Motoring in forward direction and braking in reverse direction(d) Only motoring in forward direction, no brakingAnswer: D[11] Three-phase to three-phase cycloconverter employing 18 SCRs and 36 SCRs have the same voltage and current ratings for their component thyristors. The ratio of VA rating of 36 SCR device to that of 18 SCR device is(a) 1/2(b) 1(c) 2(d) 4Answer: C[12] Three phase to three phase cycloconverter employing 18 SCRs are 36 SCRs have the same voltage and current ratings for their component thyristors. The ratio of power handled by 36 SCR device to that handled by 18 SCR device is(a) 4(b) 2(c) 1(d) 1/2Answer: A[13] The number of SCRs required for single phase to single phase cycloconverter of the mid point type and for three phase to three phase type cycloconverter are respectively(a) 4, 6(b) 8, 18(c) 4, 18(d) 4, 36Answer: C[14] For dynamic equalizing circuit used for series connected SCRs, the choice of C is based on(a) Reverse recovery characteristics(b) Turn-on characteristics(c) Turn-off characteristics(d) Rise-time characteristicsOPTIONS:1) A2) B, C3) D4) A, B, C, D5) None of the above optionsAnswer: 1[15] The function of connecting zener diode in an UJT circuit, used for the triggering of SCRs, is to(a) Expedite the generation of triggering pulses(b) Delay the generation of triggering pulses(c) Provide a constant voltage to UJT to prevent erratic firing(d) Provide a variable voltage to UJT as the source voltage changes.OPTIONS:1) B, C2) C3) A4) D5) None of the above optionsAnswer: 2Solution Hint:[4] When the conduction angle is 0, it will not suppress any part of the input, whereas during 180 degree conduction angle, it will suppress entire input waveform.[5] PMAX >= Pavg/(f.TON)Duty cycle D =TON/T = f.TONPMAX >= Pavg/D =100/0.4 =250W[7] When a SCR is in the forward blocking state, outer junctions (j1 & j3) are forward biased and middle junction(j2) is reverse biased[15] Remember that the use of zener diode in a circuit is to provide voltage regulation ( constant voltage)